Fall cankerworms: Early crop attacks Richmond area trees

For the second year in a row, the suburban Richmond area, including the counties of Chesterfield, Henrico, Hanover,Powhatan and Amelia, experienced an outbreak of fall cankerworm – a native insect pest that defoliated many forested areas, according to an e-newsletter from the Virginia Department of Forestry.

This year is complicated by the fact that the cankerworms were at least a few weeks early due to warmer temperatures and leaves on many trees were just emerging -- typically when fall cankerworm outbreaks occur most leaves are already fully emergent.

Though these insects presented no health problems to the public, some homeowners with heavy defoliation around their property were distressed by the sudden appearance of defoliation; the large degree of webbing, and the insect excrement (called frass) that fells from the trees and covered everything.

VDOF’s Forest Health Specialist Dr. Chris Asaro said, “As for the trees, even those that were completely defoliated usually recover by leafing out again. Since fall cankerworm is an early spring defoliator, trees that are otherwise healthy will have time to refoliate and green up again by mid-summer.”

Cankerworm outbreaks can re-occur over several years, but typically populations crash on their own as they are attacked by numerous insect predators and parasites. Most trees with only one year of defoliation or less than 50 percent defoliation over multiple years will recover – provided that drought or some other environmental stress factor is not also present.

“In most circumstances, spraying an insecticide, such as B.t., is not desirable or practical over such a large scale, although some communities with chronic fall cankerworm infestations have opted to have trees treated,” Asaro said. “With the current outbreak, however, most of the feeding had already occurred so spraying at this point would serve no purpose.”

Fall cankerworm caterpillars hatch in late March or April over their range, and in eastern Virginia will quickly be approaching full size.

“While cankerworm outbreaks are unpredictable, they generally recur in the same areas,” Asaro said. “It is difficult to predict how this year’s outbreak will play out given the unusual timing of things. Hopefully, not enough emergent leaf material will be present to support these populations and many caterpillars will starve. Either that or it will be harder for them to find food, and by spending more time searching for food they further expose themselves to the elements and natural enemies such as birds and other insects.”